Authorities say Dzhokhar Tsarnaev is beginning to respond in writing to questions from federal interrogators, though plenty of questions still remain. NBC’s Pete Williams reports.

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>>>begun to speak with the surviving
boston marathon
bombing suspect. even though he's hospitalized and under arrest and his brother, an alleged accomplice is dead, officials are urgently working to determine if there was more to this terror plot than it appears. tonight as prosecutors prepare to file
formal charges
against dzhokhar satsarnaev,
pete williams
has the latest on the investigation.

>> reporter: lester, good evening. nearly
48 hours
after the arrest, a team of
federal investigators
had hoped by now that they'd be talking to dzhokhar tsarnaev asking him what happened and why. there's been no such conversation yet but tonight
law enforcement
officials say they are getting some responses. though he lies under guard in a boston hospital in serious condition with gunshot wounds, including one to the throat, federal officials say dzhokhar tsarnaev is beginning to respond to a few questions from a team of
federal investigators
. he's being questioned for brief periods, officials say, because of his condition and he's responding mostly in writing but he has difficulty speaking because of his throat wound which some investigators say could be the result of a suicide attempt. newly released video from
police helicopter
shows how he was arrested. an
armored vehicle
with a robotic arm tore away a tarp covering a back on a backyard trailer where he was hiding friday night. in this thermal image, the heat from his body clearly shows him lying in the boat. agents threw in small stun grenades and a short time later he was under arrest.
police radio
relaid the word. boston
transit police
actually put the handcuffs on his wrists, taking on that duty to honor richard donahue, the
transit police
officer wounded thursday night in a shoot outwith the tsarnaev brothers. among questions unanswered so far -- where were the bombs built and did the bombers test explosives somewhere outside of town, did they have help. so far investigators say the answer seems to be no. where did they get their guns? investigators are working to trace that now. what were they doing after the bombings? dzhokhar tsarnaev classmates say he returned to the dartmouth campus where he was a student. did the trip to dagestan have anything to do with the bombing or his radicalization? as for the
boston marathon
bombings themselves, those who have seen a still unrelated surveillance video acquired by the fbi say it shows dzhokhar tsarnaev removing his backpack at the spot where the second bomb was placed and acting very differently from those around him when the first bomb went off.

>>it does seem to be pretty clear that this suspect took the backpack off, put it down, did not react when the first explosion went off and then moved away from the backpack in time for the second explosion. so pretty clear about his involvement and pretty chilling, frankly, as it was described to me.

>>and this one other note -- police say that the brothers carjacked an suv thursday night and held the car's owner at gunpoint. he tells nbc news tonight that he escaped when they stopped at a
gas station
. he described them as brutal and cautious. he asked that we not identify him and he declined an on-camera interview.

>>pete, it is our understanding
criminal charges
are being prepared as we speak but there are some on
capitol hill
who want to warm this up to an
act of war
. what other options are there in terms of charging this young man?

>>well, he's going to be charged in civilian court. even the republicans on -- in congress say that they understand that. what they want the government to do is declare him an enemy combatant simply for the purpose of questioning, and then turning him back over to civilian authorities for the trial. but tonight the
justice department
says that's not going to happen.